This invention relates to a process control and more particularly to a method and apparatus for controlling the heating of an electric element. The teaching of this invention is particularly applicable for controlling electric heating of windings on electrical equipment.
Prior art methods of monitoring and controlling temperatures of electric coils being heated have utilized thermistors, thermocouples, thermometers or other temperature sensing devices disposed in close proximity to the windings. A device which normally provides for continuous control of the sensed temperature have disadvantages when used for controlling the temperature of an electrically heated winding. Since the continuous sensing device must have a temperature sensor near the winding this necessitates a specially constructed winding or embedding the sensor in the insulation. When the winding is disposed in a solid insulation, the prior art temperature sensors also have to be contained in the insulation close to the winding. This is a complicated and expensive procedure. When a temperature sensor is embedded in the solid insulation surrounding, say a stator winding, there is a possibility of problems during later operation of the electric apparatus.